Ok so apparently Russia and Ukraine are shooting at each other in the Kerch Strait. Let me give you a quick rundown on what that's about <THREAD>
Russia and Ukraine have been essentially at war for nearly 5 years. In 2014, after the Putin-backed President of Ukraine Viktor Yanukovych put the brakes on Ukraine joining the EU, he was thrown out of office in a revolution and chased out of the country.
Russia, having lost their friendly President, decided to do what they often do, and make Ukraine too unstable to be considered a candidate for joining the EU. They annexed Crimea, invaded eastern Ukraine, and according to investigations, shot down MH17. 
Having annexed Crimea, Russia set about connecting the peninsular up to Russia proper, by building power lines and a bridge across the Kerch Strait.
The Kerch Strait is very important for both Russia and Ukraine. It links the Black Sea with the Sea of Azov, a very shallow body of water which is where some important Russian and Ukrainian ports are.
Russia, as it now controls both sides of the Kerch Strait now, would very much like to control who goes in and out of the Sea of Azov, and Ukraine doesn't want that at all. Hence, it's been a bit tense for some time.
Overnight, Russia says Ukraine sailed boats into their territorial waters, and Ukraine says Russia responded by ramming and shooting their boats. Russia blocked the Kerch Strait by parking a big boat under the only navigable span of their new bridge.
The Kerch Strait now has big queues on either side. This image is from @MarineTraffic. The boats marked in the middle are 20+ hour old readings. Nobody is traveling through the Strait.
The @AtlanticCouncil reports that over the last several months, Russia has been bothering Ukranian merchant vessels as they go through the Kerch Strait:
The European Union has come to Ukraine's defense - in writing at least. eeas.europa.eu/headquarters/h…
It's probably not a coincidence that this is happening only days before the meeting between Donald Trump and Vladimir Putin at the G20 in Argentina.
It could be an attempt from Ukraine to stop that meeting, or an attempt by Russia to solidify control over the Kerch Strait, by inciting violence and waiting to see whether Trump will condemn Putin.
Ukrainian President Poroshenko has reportedly suggested imposing Martial Law in response to this incident, only 4 months out from an election which he is likely to lose to an even more anti-Russia candidate.
UN Security Council meeting called. US reaction to this is crucial to how it will play out.
The UN Security Council meeting went pretty much as expected, with Western powers backing Ukraine's version of events, and Russia disputing it, saying that their actions are justified because the Ukrainian boats violated their territorial waters.
Meanwhile Ukrainian President Petro Poroshenko has imposed Martial Law, saying that he believes Russia is planning an invasion. He's disputing Russia's argument that he's doing it to try and delay next year's Presidential Election.
The declaration of Martial Law isn't nationwide - it covers the regions with a border with Russia or Russian-backed Transnistria (a breakaway region of Moldova). It also covers the waters around Crimea.
Meanwhile Russia has re-opened the Kerch Strait for merchant vessels.
